Electronic microcircuits have enjoyed phenomenal growth since their introduction in the 1960's, and are used in a wide range of applications.
Electronic microcircuits, often referred to as "chips", may be either integrated circuits, where each individual circuit element is fabricated on a single piece of semiconductor material, or hybrid microcircuits, in which individual electronic components, including integrated circuits, are mounted on a substrate and interconnected by conductors fabricated on the surface of the substrate or by wires or both. Although the present invention will be illustrated in the context of integrated microcircuits, it should be recognized that the invention is applicable to both integrated microcircuits and hybrid microcircuits.
Integrated microcircuits, or "chips", are typically manufactured in arrays of large numbers of individual circuits on a wafer of semiconductor material. After manufacture, individual chips are separated from the wafer and are mounted on a substrate. The substrate supports the individual chip and the leads which enable the microcircuit to be connected to a circuit board. The microcircuit and the substrate may be encapsulated in a plastic or similar material.
The substrate is typically made of either plastic or ceramic material. Ceramic material has a number of well-known advantages over plastic. A ceramic substrate can be hermetically sealed, whereas a plastic substrate cannot. Ceramic can also withstand higher temperatures than plastic. Another advantage of a ceramic substrate is that during reflow soldering, the device can better withstand thermal mismatch because the ceramic expands at a predictable rate. The leads can be designed to yield to compensate for the differences in thermal expansion between a ceramic substrate and its plastic package.
Ceramic substrates are typically made out of aluminum oxide (alumina). However, other ceramic materials possessing special properties may be used. For example, beryllium oxide (beryllia) may be used when superior heat conductivity is required, titania may be used where high dielectric strength is required, and black ceramics may be used where no light emission or penetration is desired.
Although different ceramic materials offer different properties in one way or another, all ceramic materials share a common drawback. Ceramic can withstand very large compression forces but practically no tensile forces. This disadvantage has made it extremely difficult to side bond leads to ceramic substrates using thermocompression bonding or thermosonic bonding. In order to bond leads to the sides of ceramic substrates, it has been necessary in the past to braise the leads to the substrate. However, braising has the disadvantage of requiring high temperatures, which in turn limits the kinds of materials that can be used for the leads. For example, copper cannot be used because it oxidizes at braising temperatures.
The present invention makes it possible to thermocompression bond leads to the sides of a ceramic substrate. It is thus no longer necessary to braise the leads to the sides of the substrate. Copper leads can now be used, rather than more exotic alloys.